At the Meeting #90 of the Third Generation Partnership Project Service and Systems Aspects Working Group 2 (3GPP SA WG2), a work item for Study on Optimized Offloading to WLAN in 3GPP-RAT mobility (WORM) has been approved to focus on investigate routing optimization by moving one or more Internet Protocol (IP) flows of a User Equipment (UE) to a Wireless Local Area Network (WLAN) access according to an Access Network Discovery and Selection Function (ANDSF) policy (relative priorities of a 3GPP access and the WLAN access) during the 3GPP inter-RAT handover. The work item is concerned with the technologies of the 3GPP inter-RAT handover of the UE, IP flow mobility and the ANDSF, an introduction of which will be given below respectively.
(I) 3GPP Inter-RAT Handover of a UE
A 3GPP access refers to a radio access network, e.g., a UTRAN, an E-UTRAN, etc., deployed with an access technology defined by the international standardization organization 3GPP. A 3GPP access device refers to a radio access device, e.g., a Node B, an evolved Node B (eNB), etc., manufactured as per the specification defined by the 3GPP; and a source 3GPP access device refers to a 3GPP access device with which the UE is connected before handover. 3GPP inter-RAT handover of UE has been introduced in details in the 3GPP TS 23.401, and this procedure will be introduced below in details by way of an example where a UE is handed over from an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) to a Universal Terrestrial Radio Access Network (UTRAN) Iu mode:
The procedure in which the UE is handed over from the E-UTRAN to the UTRAN Iu mode includes a handover preparation phase and a handover execution phase, and the handover preparation phase as illustrated in FIG. 1 includes:
In the step S101, a source eNB initiates a procedure of 3GPP inter-RAT handover so as to hand the UE over to a target access network, i.e., the UTRAN Iu mode;
In the step S102, the source eNB sends a Handover Required message to a source Mobility Management Entity (MME) to request allocation of a resource for the UE in a target Radio Network Controller (RNC), a target Serving General Packet Radio Service (GPRS) Support Node (SGSN) and a serving gateway (GW);
In the step S103, the source MME sends a Forward Relocation Request message to the target SGSN to initiate a handover resource allocation procedure;
In the step S104, the target SGSN determines whether to relocate the serving GW for the UE, and if the serving GW needs to be relocated, then the target SGSN sends a Create Session Request message to the target serving GW per Packet Data Network (PDN) connection;
In the step S104a, the target serving GW allocates its local resource for the PDN connection and returns a Create Session Response message to the target SGSN;
In the step S105, the target SGSN sends a Relocation Request message to the target RNC to request the target RNC for allocation of radio resource for the UE;
In the step S105a, the target RNC allocates resource and returns to the target SGSN a Relocation Request Acknowledge message carrying information on Radio Access Bearers (RABs) established by the target RNC;
In the step S106, if Indirect Forwarding and Direct Tunnel is adopted and a serving GW is relocated, then the target SGSN sends a Create Indirect Data Forwarding Tunnel Request message to the target serving GW;
In the step S106a, the target serving GW returns a Create Indirect Data Forwarding Tunnel Response message to the target SGSN;
In the step S107, the target SGSN sends a Forward Relocation Response message to the source MME;
In the step S108, if Indirect Forwarding is adopted, the source MME sends a Create Indirect Data Forwarding Tunnel Request message to the serving GW used for Indirect Forwarding; and
In the step S108a, the serving GW returns a Create Indirect Data Forwarding Tunnel Response message to the source MME.
The target network side allocates the desired resource for the UE at the end of the handover preparation phase, and the UE is handed over from the E-UTRAN to the UTRAN Iu mode in the handover execution phase as illustrated in FIG. 2 including:
In the step S201, the source MME sends a Handover Command message to the source eNB at the end of the handover preparation phase;
In the step S202, the source eNB sends an HO from E-UTRAN Command message to the UE to instruct the UE to execute a handover to the target access network;
In the step S203, the UE moves to the target UTRAN Iu (3G) system and executes the handover according to parameters in the HO from E-UTRAN Command message, which is a UTRAN Iu access procedure;
In the step S204, the target RNC sends a Relocation Complete message to the target SGSN after the UE and the target RNC exchange successfully an RNC-ID and a Serving-Radio Network Temporary Identifier (S-RNTI). This message indicates that the UE has moved from the E-UTRAN to the RNC;
In the step S205, after the UE has arrived to the target side, the target SGSN sends a Forward Relocation Complete Notification message to the source MME to inform the source MME that the UE has arrived to the target side, and the source MME starts a timer upon reception of the message;
In the step S206, the target SGSN starts a timer upon reception of a Forward Relocation Complete Acknowledge message;
In the step S207, the target SGSN sends a Modify Bearer Request message to the target serving GW to complete the handover procedure;
In the step S208, the target serving GW sends a Modify Bearer Request message to a Packet Data Network (PDN) gateway (GW) to modify a context of the UE;
In the step S208a, if the serving GW is relocated, then the PDN GW updates the context of the UE and returns a Modify Bearer Response message to the target serving GW;
In the step S209, the target serving GW sends a Modify Bearer Response message to the target SGSN to indicate that the user plane switch procedure has been completed; and
In the step S210, when the UE recognizes that its current Routing Area is not registered with the network, or when a Temporary Identity used in Next update (TIN) of the UE indicates a Globally Unique Temporary Identifier (GUTI), the UE initiates a Routing Area Update (RAU) procedure;
In the step S211, after the timer started by the source MME in the step S205 expires, the source MME sends a Release Resource message to the source eNB, and if the serving GW serving the UE is changed, then the source MME sends a Delete Session Request message to the source serving GW. The source serving GW deletes the context of the UE and then sends a Delete Session Response message to the source MME;
In the step S212, if Indirect Forwarding is used, then after the timer started by the source MME in the step S205 expires, the source MME sends a Delete Indirect Data Forwarding Tunnel Request message to the source serving GW to release the temporary resource allocated by the source serving GW used for Indirect Forwarding; and
In the step S213, if Indirect Forwarding is used and the serving GW is relocated for the UE, then after the timer started by the target SGSN in the step S206 expires, then the target SGSN sends a Delete Indirect Data Forwarding Tunnel Request message to the target serving GW to release the temporary resource allocated by the target serving GW used for Indirect Forwarding.
(II) IP Flow Mobility
The UE is connected to an EPC through a WLAN access in three different network architectures including an S2a interface based architecture, an S2b interface based architecture and an S2c interface based architecture. S2a interface locates between a trusted WLAN access and GW in EPC, S2b interface locates between an un-trusted WLAN access and GW in EPC, S2c interface locates between UE and GW in EPC. Because the locations of the three interfaces are different, so the procedure to move an IP flow to the WLAN access is different in different architectures. Whether a WLAN access is trusted or not is decided by the operator.
IP Flow Mobility (IFOM) enables IP flow mobility between a 3GPP access and a WLAN access, including a process of seamless offloading to a WLAN, mobility of an IP flow between the 3GPP access and the WLAN access, and interaction of a PCC with an ANDSF. This process has been described in details in the 3GPP TR 23.861.
The UE has established a PDN connection through the 3GPP access, and after the UE finds the WLAN access, the UE can add the WLAN access to the PDN connection. A particular process thereof is as illustrated in FIG. 3 which illustrates a method of adding a WLAN access to a PDN connection including:
In the step S301, the UE searches for, finds and is connected with the WLAN access and configures an IPv4 address and/or an IPv6 address/prefix;
In the step S302, the UE performs functions of discovering a Home Agent (HA), bootstrapping the Dual-Stack Mobile IPv6 (DSMIPv6) and monitoring a DSMIPv6 home link, and reference can be made to the TS 23.402 for details thereof;
In the step S303, the UE transmits a Dual-Stack Mobile IPv6 (DSMIPv6) Binding Update message to the HA over the WLAN. The UE can carry a plurality of routing rules in the message. The DSMIPv6 Binding Update message carries an indicator that it is still connected over the home link (i.e., through the 3GPP access), and a binding identifier mobility option in the message describes relevant binding to the home link and other binding to a forward address from the WLAN access;
In the step S304, if the home agent of the UE functions on the PDN GW and dynamic PCC is deployed, then the PDN GW transmits an IP Connectivity Access Network (IP-CAN) Session Modify Request message to a Policy and Charging Rule Function (PCRF) by carrying an updated routing rule in the request message;
In the step S305, if the home agent of the UE functions on the PDN GW, then the PCRF transmits an acknowledgment message including an updated PCC rule (if the PCC rule is updated) to the PDN GW according to resource established successfully in a Bearer Binding and Event Reporting Function (BBERF);
In the step S306, the HA creates DSMIPv6 binding, is configured with the IP flow routing rule and returns a Binding Acknowledge message to the UE to indicate to the UE that the routing rule requested by the UE has been accepted;
In the step S307, the PCRF acknowledges the configuration of a Quality of Service (QoS) rule relevant to Service Data Flows (SDF's) in the target BBERF in response to the IP-CAN Modify Request, which is a procedure performed in a gateway control session and a QoS rule provision procedure, and reference can be made to the TS 23.203 for details thereof; and
In the step S308, if the home agent of the UE functions on the PDN GW, then a resource allocated by the source 3GPP access system for the IP flow moved to the WLAN access is removed in a procedure of releasing a 3GPP resource.
FIG. 4 illustrates an IP flow mobility process in a PDN connection including:
In the step S401, the UE is connected with both the 3GPP access and the WLAN access, and all of current traffic of the UE is forwarded from the UTRAN Iu mode whereas no traffic is passed through the WLAN access;
In the step S402, the UE transmits a binding update message to the HA to configure a new routing rule or to modify an existing routing rule so as to apply an IP flow specific to a WLAN access route;
In the step S403, when the HA is deployed to function in the EPC, the PDN GW transmits an IP-CAN Session Modify Request message including an updated routing policy to the PCRF, and mapping between routing addresses and SDF's is stored in the PCRF;
In the step S404, if the HA is deployed to function in the EPC, then the PCRF transmits an acknowledgment message including an updated Policy and Charging Control (PCC) rule to the PDN GW according to resource established in the WLAN
In the step S405, the HA transmits a Binding Acknowledge message to indicate to the UE that the routing rule requested by the UE has been accepted;
In the step S406, if the HA is deployed to function in the EPC, then the PCRF makes sure in response to the IP-CAN Session Modify Request that the target RAT has been configured with the relevant QoS rule and the relevant QoS rule has been removed from the source RAT. With a non-3GPP access, this procedure is performed in a Gateway (GW) control session and a QoS rule provision procedure. Reference can be made to the 3GPP TS 23.203 for the GW control session and the QoS rule provision procedure; and
In the step S407, if the HA is deployed to function in the EPC, then a procedure of releasing an Evolved Packet System (EPS) resource is performed to release the resource from the 3GPP source access system. Reference can be made to the 3GPP TS 23.402 for details thereof.
(III) An ANDSF
A user equipment for which multiple access systems are supported can be provided by a network with information about available radio access systems, and this function is performed by the ANDSF in the network. FIG. 5 illustrates the architecture of discovering and selecting an access network in a non-roaming scenario, where the ANDSF is a home ANDSF (H-ANDSF).
The user equipment communicates with the ANDSF via an S14 interface. With the ANDSF, the UE can know some information about an access network. The user equipment communicates with the ANDSF through pulling and pushing, where in the former case, the UE requests the ANDSF for the relevant information on its own initiative, and in the latter case, the ANDSF pushes the relevant information to the user equipment on its own initiative. The ANDSF can provide the UE with an Inter-System Mobility Policy (ISMP), an Inter-System Routing Policy (ISRP), Access Network Discovery Information (ANDI) and other information.
Particularly the ISMP relates to information about a series of operator defined rules and preferences, where the policy defines whether to allow inter-system mobility, the most appropriate type of RAT for an access to an Evolved Packet Core (EPC), different priorities of different access schemes and other information. The ISRP includes some information required for inter-system routing, and a user equipment for which multiple radio access interfaces are supported, e.g., a user equipment for which IP Flow Mobility (IFOM) or Multiple-Access PDN Connectivity (MAPCON) is supported, can decide, under the ISRP policy, over which of available access networks, data is transmitted when a specific routing condition is satisfied; and when some access network is disabled for a specific IP data flow and/or a specific APN. The ANDI includes a list of access networks, available nearby the position where the UE is located, of a requested type of RAT, and relevant parameters thereof, e.g., the type of RAT, the identifiers and carrier frequencies of the radio access networks, etc.
The ISRP policy has been extended by the WORM task force and will be introduced below in details. The ISRP policy includes one or more ISRP rules including IP flow description information, routing conditions (rule valid positions and rule valid periods), routing rules and rule priorities. When the current position of a UE and the current time satisfy the routing condition or conditions of one or more ISRP rules, the UE selects an ISRP rule at the highest one of the priorities of these rules. Then the UE selects one of the access networks under the routing rule in the ISRP rule, accesses the access network and moves IP flows, transmitted by the UE, matching with the IP flow description information of the ISRP rule, to the access network.
A routing rule in an ISRP rule includes an access technology, an RAT identifier, a secondary RAT identifier and the priority of an access network. A format of the routing rule is as illustrated below, where the access technology includes a Global System for Mobile Communication (GSM) Enhanced Data rates for GSM Evolution (EDGE) Radio Access Network (GERAN), a Universal Terrestrial Radio Access Network (UTRAN), an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and a WLAN. Only if the access technology is a WLAN, then the routing rule will include the RAT identifier, the secondary RAT identifier and other information, where the SSID of the WLAN is stored in the RAT identifier, and the HESSID of the WLAN is stored in the secondary RAT identifier. The priority of the access network represents the priority of the access technology in the routing rule.
In summary, there are the following problems during the 3GPP inter-RAT handover of a UE:
Since there are such a limited resources of the target 3GPP access system that one or more bearers of the UE may be lost or the QoS of thereof may be degraded, there may be a poorly guaranteed QoS of IP flows over these affected bearers, thus discouraging the experience of the user and consequently possibly interrupting the services; and after the UE is handed over from the source 3GPP access to the target 3GPP access, the UE moves the specific IP flow to the WLAN access under the policy (e.g., the ANDSF policy) so that two handovers occurs.